CBCB (Compact Binary Coalescence... and Blues)

Last week, I told you about my work on gravitational waves that went into my PhD. My post-doc is still in the same field, but I'm looking for an entirely different type of wave than before. The continuous waves I studied before were weak, but long lasting. The work I do now is on waves from compact binary coalescences, CBCs. These are among the most powerful waves we expect to see, but last only seconds. All the detections LIGO and Virgo have made so far were from CBCs.

The name "Compact Binary Coalescence" is awfully jargony, so let me take it piece-by-piece:
Compact: A compact object is one with unusually high density. In this case it refers to either a neutron star, or a black hole. How high is the density? One teaspoon of neutron star material weighs 10 million tons, and black holes are even denser!
Binary: A binary system has two objects that orbit their common center of gravity. If one object is significantly bigger than the other, it might look like the small one orbits the big one, but even our Sun has a bit of wobble from the pull of the surrounding planets.
Coalescence: As the objects orbit, they radiate energy in the form of gravitational waves. That energy comes from their gravitational potential, which draws them closer together as it decreases. A closer orbit is also a faster one (think of Mercury), so the pair radiate stronger waves, which further speeds the process. Eventually, the two collide and coalesce into a single object.

I can show you what this type of orbit looks like thanks to the recently (but not yet officially) released LIGO Orrery:

This was inspired by the Kepler Orrery released a few years ago. It shows simulations of all the binary black hole detections LIGO and Virgo have made so far. Below the orbits, you can see a plot of the gravitational wave signal. Each peak of the wave corresponds to a half-orbit of the bodies. As the rate of revolution increases, so does the frequency of the wave.

The signal is divided into three parts
Inspiral: The majority of time is spent in this phase, as the orbit decays and the bodies spiral inward (good name, right?). The wave is sinusoidal with increasing frequency and amplitude.
Merger: When the bodies first collide, the new object is oblong, and takes some time to smooth out. This is the peak wave output.
Ringdown: There are tremendous forces involved in the collision, so the black hole that results from the merger needs to shake off the extra energy. You can't see it in the plots shown here, but immediately after the big burst are a couple more small waves.

I'm working with a group here in Annecy, France that's part of the European gravitational wave collaboration, Virgo. We look for these CBC signals in the data coming from the two LIGO detectors in the US, and the Virgo detector in Italy. The search technique I'm part of is called MBTA, which is not the Massachusetts Bay Transit Authority, but the Multi-Band Template Analysis. The goal is to identify signals as fast as possible, so we can send the coordinates to partner groups observing in the electromagnetic spectrum. This allowed us to show last year that gamma ray bursts are associated with neutron star mergers.

I have a lot to learn in this new pursuit, but everyone at my office and in town have been incredibly welcoming. I've been given an amazing opportunity to live an work in such a beautiful part of the world!